The invention relates to a method of manufacturing a picture display tube in which a gettering device is mounted in the tube. The gettering device comprises a metal holder in which are accommodated a source of evaporable gettering metal and a gas source of a material which releases gas upon heating. After evacuating the tube, the gettering device is heated inductively to release the gas from the gas source and to evaporate the gettering metal from the source of gettering metal.
The invention further relates to a picutre display tube thus manufactured, as well as to a gettering device suitable for use in the above-mentioned method.
Such a method is disclosed in U.S. Pat. No. 3,388,955. In this method the metal holder of the gettering device comprises a gettering metal to be evaporated which is mixed with a gas source of gasreleasing material. During the inductive heating of the gettering device, first the gas is released from the gas source and the gettering metal is then evaporated. The gettering metal is evaporated in a gas atmosphere to obtain a uniform distribution of gettering metal on an inner surface of the display tube.
The known gettering device comprises a gas source consisting of iron nitride powder (Fe.sub.4 N) which is mixed with the source of gettering metal in powder form. In such a gettering device the iron nitride is attacked by moist air of approximately 450.degree. C., which condition is met, for example, in the manufacture of a colour television display tube when the display window and the cone of the display tube are sealed together by means of a sealing glass. The known gettering device may not be installed in the tube before the display window and the cone are secured together. This is a serious restriction inter alia in the manufacture of a colour display tube having a resistive layer provided internally on a part of the tube wall, as described in British patent specification No. 1,226,728. The resistive layer is provided near the neck-cone transition of the tube, which makes it necessary for the gettering device to be disposed in the tube in a place remote from the neck-cone transition, to avoid forming an electric shortcircuit on the resistive layer by means of gettering metal evaporated from the gettering device. Because of its inaccessibility after the window is secured, it is desirable to install the gettering device in a place remote from the cone-neck transition before the cone is secured to the window of the tube. Such installation is an improvement over the typical insertion of the gettering device by means of a resilient metal strip attached to the gun system disposed in the neck of the tube thereby avoiding the resilient force exerted on the gun system by the metal strip.
A gas source which can be exposed to moist air of at least 450.degree. C. without any adverse effects is described in British patent specification No. 1,405,045. In this Specification the gas source comprises a germanium nitride, in particular Ge.sub.3 N.sub.4, as a gas-releasing material. Germanium nitride is a stable compound having a decomposition temperature which is comparatively high with respect to iron nitride. This has for its result that upon heating the gettering device, the gas source accommodated therein gives off its nitrogen only during the evaporation of the gettering metal. In order to obtain a layer of gettering metal which is porous through-out its thickness and hence is readily absorbing on an internal surface of the tube, it is necessary that during the heating of the gettering device the gas released from the gas source has already built up a sufficient pressure of approximately 133.times.1O.sup.-3 to 666.times.1O.sup.-2 Pa in the tube before the gettering metal begins to evaporate.